The prior art discloses many different types of refractory structures used to protect a variety of structures including hot air ducts, hot water and steam pipes from heat loss. Silicon carbide tube blocks of various configurations are in use to protect fluid carrying tubes in incinerators and circulating fluid bed boilers. As used herein, "boiler" is used generically to refer to all high temperatures boiler assemblies including incinerators, power generating boilers, and the like. Although the prior tube block shapes have relatively high thermoconductivity, ultimate heat flow is not achieved in prior art designs because such tube blocks are not in sufficient direct contact with the metal fluid carrying tubes. In most of these systems, some type of silicon carbide mortar is used between the tubes and the refractory tube block to fill any air gaps and enhance heat flow. However, because of the lower thermal conductivity of these mortars and the thickness of the mortar layer, the heat flow is far below optimum. In other cases, manufacturing variances of the fabricated metal tube walls do not allow the refractory tube blocks to be manufactured with the exacting tolerances required for the intimate fit desired. The resulting air gaps negatively effect the heat flow between the tubes and the tube block assemblies.
Maximum heat flow is critical to boiler efficiency. If the heat transfer between the refractory tube block assembly and the boiler tubes is less than optimum, the outer surface of the tube blocks becomes hotter than designed. As the temperature increases, ash from the fuel being burned will cling to the surface and form an insulating layer. Once this phenomenon begins, the layer gets increasingly thick until heat transfer becomes extremely poor. The "flue gas" above the combustion zone then increases in velocity and temperature, often above the design limits, and causes corrosion/erosion problems downstream in the boiler. In addition, the layer of ash may eventually break off as it grows and cause damage to the base of the boiler.
None of the prior art tube block structures overcome these problems.